Ultrasonic medical imaging plays a crucial role in modern medicine, gradually becoming more and more important as new developments enter the market. One of the most common ultrasound imaging applications is echocardiography, or ultrasonic imaging of the cardiac system. Other widespread applications are obstetrics and gynecology, as well as abdominal imaging, to name a few. Ultrasonic imaging is also used in various other industries, e.g., for flaw detection during hardware manufacturing.
Ultrasonic imaging systems typically produce relatively noisy images, making the analysis and/or diagnosis of these images a task for highly trained experts. One of the most problematic imaging artifacts is clutter, i.e., undesired information that appears in the imaging plane, obstructing data of interest.
One of the main origins of clutter in ultrasonic imaging is effective imaging of objects outside the probe's mainlobe, also referred to as sidelobe clutter. For example, in echocardiography, the dominant reflectors outside the probe's mainlobe are typically the ribcage and the lungs.
Another origin of clutter is multi-path reflections, also called reverberations. In some cases, the geometry of the scanned tissue with respect to the probe, as well as the local reflective characteristics of the tissue, causes a noticeable percentage of the transmitted energy to bounce back and forth in the tissue before reaching the probe. As a result, the signal measured for a specific range with respect to the probe may include contributions from other ranges, in addition to the desired range. If the signal emanating from other ranges is caused by highly reflective elements, it may have a significant effect on the image quality.
A common method for enhancing the visibility of the desired ultrasonic image relative to the clutter, particularly in patients with low echogenicity (a common phenomenon among obese patients), is administering contrast agents. Such agents enhance the ultrasonic backscatter from blood and aid in its differentiation from the surrounding tissue. This method is described, for example, by Krishna et al., in a paper entitled “Sub-harmonic Generation from Ultrasonic Contrast Agents,” Physics in Medicine and Biology, vol. 44, 1999, pages 681-694, which is incorporated herein by reference.
Using harmonic imaging instead of fundamental imaging, i.e., transmitting ultrasonic signals at a certain frequency and receiving at twice the transmitted frequency, also reduces clutter effects. Spencer et al. describe this method in a paper entitled “Use of Harmonic Imaging without Echocardiographic Contrast to Improve Two-Dimensional Image Quality,” American Journal of Cardiology, vol. 82, 1998, pages 794-799, which is incorporated herein by reference.
U.S. Pat. No. 6,251,074, by Averkiou et al., issued on Jun. 26, 2001, titled “Ultrasonic Tissue Harmonic Imaging,” describes an ultrasonic diagnostic imaging system and methods, which produce tissue harmonic ultrasonic images from harmonic echo components of a transmitted fundamental frequency. Fundamental frequency waves are transmitted by an array transducer to focus at a focal depth. As the transmitted waves penetrated the body, the harmonic effect develops as the wave components begin to focus. The harmonic response from the tissue is detected and displayed, while clutter from fundamental response is reduced by excluding fundamental frequencies.
Furthermore, image-processing methods have been developed for detecting clutter-affected pixels in echocardiographic images by means of post-processing. Zwirn and Akselrod present such a method in a paper entitled “Stationary Clutter Rejection in Echocardiography,” Ultrasound in Medicine and Biology, vol. 32, 2006, pages 43-52, which is incorporated herein by reference.
Other methods utilize auxiliary receive ultrasound beams. In U.S. Pat. No. 8,045,777, issued on 25th Oct. 2011, titled “Clutter Suppression in Ultrasonic Imaging Systems,” Zwirn describes a method for ultrasonic imaging, comprising: transmitting an ultrasonic radiation towards a target; receiving reflections of the ultrasonic radiation from a region of the target in a main reflected signal and one or more auxiliary reflected signals, wherein each one of the reflected signals is associated with a different and distinct beam pattern, wherein all of the reflected signals have an identical frequency; determining a de-correlation time of at least one of: the main reflected signal and the one or more auxiliary reflected signals; applying a linear combination to the main reflected signal and the one or more auxiliary reflected signals, to yield an output signal with reduced clutter, wherein the linear combination comprises a plurality of complex number weights that are being determined for each angle and for each range within the target tissue, wherein each complex number weight is selected such that each estimated reflection due to the clutter is nullified, wherein a reflection is determined as associated with clutter if the determined de-correlation time is above a specified threshold.
U.S. patent application 2009/0141957, by Yen and Seo, published on Jun. 4, 2009, titled “Sidelobe Suppression in Ultrasound Imaging using Dual Apodization with Cross-Correlation,” describes a method of suppressing sidelobes in an ultrasound image, the method comprising: transmitting a focused ultrasound beam through a sub-aperture into a target and collecting resulting echoes; in receive, using a first apodization function to create a first dataset; in receive, using a second apodization function to create a second dataset; combining the two datasets to create combined RF data; calculating a normalized cross-correlation for each pixel; performing a thresholding operation on each correlation value; and multiplying the resulting cross-correlation matrix by the combined RF data.
An additional class of currently available methods for handling clutter is a family of clutter rejection algorithms, used in color-Doppler flow imaging. These methods estimate the flow velocity inside cardiac chambers or other blood vessels and suppress the effect of slow-moving objects; assuming that the blood flow velocity is significantly higher than the motion velocity of the surrounding tissue. These methods are described, for example, by Herment et al. in a paper entitled “Improved Estimation of Low Velocities in Color Doppler Imaging by Adapting the Mean Frequency Estimator to the Clutter Rejection Filter,” IEEE Transactions on Biomedical Engineering, vol. 43, 1996, pages 919-927, which is incorporated herein by reference.